The present invention relates to a new and improved device for monitoring the weft thread travel on an air jet weaving machine which is provided with a lathe or sley beam and a reed fixed thereon, an air jet for inserting the weft thread, and weft thread receiving means comprising suction means, said device comprising an optoelectrical weft thread sensor including a measuring area or field and being arranged at the reed such that the measuring area is crossed by the path of the weft thread travel.
In United Kingdom Pat. No. 1,432,422 there is disclosed a monitoring device of the aforementioned type comprising an optoelectrical sensor which is not designed for surveying the yarn travel, however responds, prior to the commencement of the read beat-up, when the foremost or leading free end of the filling or weft thread being inserted into the weaving shed crosses the measuring area or field of the sensor. Here, the criterion of a correct filling insertion is the signal difference produced by the weft thread entering the sensor. Thus, the presence of the weft thread rather than the weft travel is detected.
Operational experience has shown that thread presence monitoring is susceptible to trouble. In particular, false signals may be generated e.g. by fibre fly, thus simulating correct filling insertion when the weft thread is absent, and preventing the loom from being stopped.